I'm 74 kg and have used the 71 quad (wood convertible) in powered 4.0 cross off and bump'n'jump. Excellent board. Feels more like 74 litres heading out through the waves but still compact and controllable in the strong gusts. Allows for really confident driven bottom turns even in overpowered conditions and a fantastic forgiving top turn. Cranks upwind and easy to float on when not planing (for 71 litre board). I wouldn't need to go smaller than this as pretty much covers me up to 5.0. A 66 could be really fun if mostly at 4.0 end of sail range.